RECENT PROGRESS IN COLOR PHOTOGRAPHY. 
By THOMAS W. SMILLIBF, F. R. P. S. 
For nearly a hundred years some of the ablest photographic chem- 
ists have labored to produce photographs in color. 
Seebeck in 1810 ebtained some results with silver chloride. Sir 
John Herschel in 1840 found that chloride of silver on paper after 
exposure to white hght until it was colored violet, and then exposed 
to the solar spectrum, reproduced approximately the natural colors. 
In the period from 1847 to 1855 E. Becquerel obtained results so 
remarkable with chlorides on a silver plate as to attract the attention 
of the whole scientific world. 
Niepce de St. Victor in 1851 took up the work very earnestly, pro- 
ducing on silver plates treated with hypochlorite of soda not only 
the solar spectrum, but the various colors of flowers, fabrics, gems, 
peacock feathers, etc. 
Hunt, Poitvin, Zencker, M. de St. Florient, Kopp, Maxwell, and 
others also labored in this field. 
M. Carey Lea about 1882 began experimenting with the pabemocde 
of silver and obtained beautiful results, but, like all others up to that 
date, he was unable to make the colors permanent. 
In 1869 M. Ducos du Hauron published his heliochrome process. 
In this process three negatives are made, each one through a different 
color screen (the screens used were violet, green, and orange) ; these 
negatives were then printed on bichromatized gelatin films, colored 
red, blue, and yellow; the surplus color was then washed out and the 
films superposed. 
M. Charles Cros invented a similar process about the same time. 
The underlying principle of Ducos du Hauron’s and Cros’s proc- 
esses, and indeed of all color-screen photography, may be given as 
follows: 
If we divide the visible spectrum into three approximately equal 
parts we get three groups of colors, the principal tints of which are 
orange-red, green, and violet. Each one of these groups is comple- 
mentary to the other two, and the three contain all the simple rays 
41780—08——_19 231 
